Notes Class 10 Science Chapter 10 Human Eye and Colourful World

Notes For All Chapters Science Class 10 CBSE

1. The Human Eye – Structure and Function

The human eye is a vital and sensitive sense organ that enables us to see the world and its colours.

It functions like a camera, forming an image on a light-sensitive screen called the retina.

Light Path through the Eye:

• Cornea – Transparent, curved membrane on the front of the eye. Most refraction occurs here.
• Aqueous Humour – Fluid between cornea and lens that helps bend light.
• Pupil – Hole controlled by the iris, a muscular diaphragm that regulates light entry.
• Eye Lens – Crystalline, convex, jelly-like lens providing fine focusing.
• Retina – Delicate membrane with light-sensitive cells that convert light into electrical signals sent to the brain via optic nerve.

2. Power of Accommodation

Accommodation: The ability of the eye lens to adjust its focal length to see objects at different distances.

How it works:

• Distant objects: Ciliary muscles relax → lens becomes thin → focal length increases.
• Nearby objects: Ciliary muscles contract → lens becomes thick → focal length decreases.

Limits of Accommodation:

• The lens cannot decrease its focal length beyond a certain limit.
• Least distance of distinct vision (Near point): ~25 cm for a normal young adult.
• Far point: Infinity for a normal eye.

Cataract: Lens becomes milky/cloudy with age, causing partial or total vision loss. Can be corrected by surgery.

3. Defects of Vision and Their Correction

Sometimes the human eye loses its power of accommodation due to age, structural changes, or refractive issues. This leads to blurred or unclear vision for objects either near or far.
There are three main types of refractive defects of vision:

1. Myopia (Near-sightedness / Short-sightedness)

Definition:

A person with myopia can see nearby objects clearly but distant objects appear blurred.

Cause:

• Excessive curvature of the eye lens.
• Elongation of the eyeball, which increases the distance between the lens and retina.

Effect:

• The far point of the eye (normally at infinity) is shifted closer.
• The image of a distant object forms in front of the retina instead of on it.

Correction:

• Use a concave (diverging) lens of suitable power.
• This lens diverges incoming rays so that they focus on the retina instead of in front of it.

2. Hypermetropia (Far-sightedness / Long-sightedness)

Definition:

• A person with hypermetropia can see distant objects clearly but nearby objects appear blurred.

Cause:

• Eyeball too short, reducing the distance between the lens and retina.
• Focal length of the eye lens too long, preventing light from converging in time.

Effect:

• The near point (normally 25 cm) is shifted farther away.
• The image of a nearby object forms behind the retina instead of on it.

Correction:

• Use a convex (converging) lens of suitable power.
• This lens converges light rays before they enter the eye so they focus on the retina.

3. Presbyopia (Old-age Sight)

Definition:

• A defect usually occurring with ageing, where a person finds it difficult to see nearby objects clearly.

Cause:

• Weakening of ciliary muscles with age.
• Reduced flexibility of the eye lens.

Effect:

• The power of accommodation decreases, causing difficulty in reading or focusing on close objects.
• The near point gradually recedes farther away.

Correction:

Use convex lenses for reading and other near tasks.

In cases where the person also suffers from myopia, bifocal lenses are used:

• Upper part: Concave lens for distant vision.
• Lower part: Convex lens for near vision.

Contact lenses or surgical procedures may also correct this defect.

4. Refraction of Light Through a Prism

A prism has two triangular bases and three rectangular lateral surfaces inclined to each other.

Angle of Prism (A): Angle between two lateral faces.

Refraction through a Prism:

• Light bends towards the normal on entering prism (air → glass).
• Light bends away from the normal on emerging (glass → air).

Angle of Deviation (D): The angle between the original direction of incident ray and the emergent ray.

Unlike a glass slab, the emergent ray is not parallel to the incident ray.

5. Dispersion of White Light by a Prism

Dispersion: Splitting of white light into its seven component colours (VIBGYOR – Violet, Indigo, Blue, Green, Yellow, Orange, Red).

Cause: Different colours bend by different angles. Violet bends most, red bends least.

Spectrum: Band of seven colours produced by dispersion.

Newton’s Experiment:

• Used two prisms.
• First prism dispersed light.
• Second prism recombined the colours into white light → shows sunlight is made of seven colours.

6. Rainbow Formation

Rainbow: Natural spectrum formed due to dispersion of sunlight by water droplets in the atmosphere.

Process:

1. Refraction and dispersion as light enters droplet.
2. Internal reflection inside droplet.
3. Refraction again as light exits droplet.

Always formed opposite the Sun.

Can also be seen near waterfalls or fountains with sunlight behind you.

7. Atmospheric Refraction

Definition: Refraction of light by Earth’s atmosphere due to variation in air density and refractive index.

Examples:

a) Twinkling of Stars

• Starlight undergoes continuous refraction due to changing refractive index.
• Apparent position changes → light intensity varies → twinkling occurs.
• Planets don’t twinkle because they are closer and act as extended light sources.

b) Advance Sunrise and Delayed Sunset

• Sun visible about 2 minutes before actual sunrise and 2 minutes after actual sunset due to bending of sunlight.
• Apparent flattening of the Sun at sunrise and sunset is also due to atmospheric refraction.

8. Scattering of Light

Scattering: Deflection of light by particles in its path.

Tyndall Effect: Visible path of light when scattered by colloidal particles (e.g., sunlight through smoke or mist).

Dependence on Particle Size:

• Small particles → scatter blue light more.
• Large particles → scatter longer wavelengths (red/orange).

9. Blue Colour of the Sky

Molecules and small particles scatter shorter wavelengths (blue) more effectively than longer wavelengths (red).

Thus, scattered light reaching our eyes is predominantly blue.

At high altitudes or without an atmosphere → no scattering → dark sky.

10. Red Colour in Danger Signals

Red light scatters the least due to its long wavelength.

It is visible from a greater distance, hence used in danger signals and traffic lights.